Inkjet printers that perform printing by intermittently ejecting ink are known as printing apparatuses for printing images onto various types of media to be printed, including paper, cloth, and film.
With inkjet printers, ink is ejected as nozzles for ejecting ink are moved. For that reason, due to the law of inertia, the droplets of ink that are ejected travel from the nozzles to the medium to be printed as they move in the moving direction of the nozzles at the moving velocity of the nozzles. Consequently, the ink droplets land on the paper at positions that are shifted in the moving direction of the nozzles from the positions of the nozzles when the ink droplets are ejected.
Accordingly, with conventional inkjet printers, printing is carried out taking into account the shift in landing positions based on the moving velocity of the nozzle.
(1) The shift in the landing position caused by movement of the nozzles, however, is related not only to the moving velocity of the nozzles but also to the distance from the nozzles to the medium to be printed. For that reason, the amount that the landing position is shifted due to the movement of the nozzles also changes when the distance from the nozzles to the medium to be printed changes due to the thickness of the paper or curvature in the paper, for example.
Accordingly, to make the ink droplets land in correct positions, it is an object of a first invention to control the timing at which ink droplets are ejected, taking into account the distance from the nozzles to the medium to be printed.
(2) Also, if the timing of ink ejection were to be set at an earlier timing or a delayed timing with respect to a reference timing for ink ejection in accordance with the velocity at which the nozzles are moved, then calculations would become complicated. Furthermore, when the timing of ink ejection is at a fast timing that exceeds the performance of the head, printing can no longer be carried out accurately.
Accordingly, to make the ink droplets land correctly, a second invention makes the maximum velocity of the target moving velocity slower than a predetermined reference velocity.
(3) Also, a temporal lag between when the moving velocity of the nozzles is detected and the ink is ejected may result in a difference between the detected moving velocity of the nozzles and the moving velocity of the nozzles when ejecting ink. Consequently, even if variation in the landing positions is taken into account based on the detected moving velocity of the nozzles, ink does not land in correct positions when the moving velocity of the nozzles when ejecting ink is different from the detected moving velocity of the nozzles.
For example, if printing is carried out when the nozzles are accelerating or decelerating, then when there is a temporal lag between when the moving velocity of the nozzles is detected and when the ink is ejected, there would be a difference between the detected moving velocity of the nozzles and the moving velocity of the nozzles when ink is ejected. Thus, the ink will not land at correct positions when the nozzles are accelerating or decelerating simply by controlling the timing at which ink is ejected based on the detected moving velocity of nozzles, as is the case with conventional inkjet printers.
Accordingly, to make the ink land at correct positions, it is an object of a third invention to control the timing at which the ink droplets are ejected in accordance with the degree of acceleration of the nozzles.
(4) Also, when the detected moving velocity of the nozzles includes error, then the ink will land on the medium to be printed at positions shifted from the correct positions if the shift in the position where the ink droplets land is calculated based on that moving velocity including error.
In particular, when the moving velocity of the nozzles is detected based on the output of an encoder, the velocity is detected in a stepwise manner if the encoder has low resolution, and thus there is large error in the detected velocity. Moreover, if consideration to the shift in landing position of the ink droplets is given based on the detected moving velocity including large detection error, the ink will land on the medium to be printed shifted from the correct positions.
Accordingly, to make the ink land in correct positions, it is an object of a fourth invention to control the timing at which the ink droplets are ejected based on the results of a plurality of detections.